High strength transparent ceramic using corundum powder and methods of manufacture

ABSTRACT

High strength transparent corundum ceramics using corundum powder and methods of manufacture are disclosed. The method of forming transparent corundum ceramics includes milling corundum powder in aqueous slurry with beads. The method further includes processing the slurry by a liquid shaping process to form a gelled body. The method further includes sintering the gelled body in air and pressing the gelled body by hot isostatic pressing to form a ceramic body.

FIELD OF THE INVENTION

The invention relates to corundum ceramics using corundum powder and,more particularly, to high strength transparent corundum ceramics usingcorundum powder and methods of manufacture.

BACKGROUND OF THE INVENTION

Ceramics are very versatile in their industrial use ranging fromapplications in engine components, frames, etc. The properties ofceramic materials are based on many factors including, for example, thetypes of atoms, the bonding between the atoms, and the packaging of theatoms. For example, ceramics usually have a combination of ionic andcovalent bonds. The covalent bond typically results in high elasticmodulus and hardness, high melting points, low thermal expansion, andgood chemical resistance.

More specifically, due to ceramic materials wide range of properties,they can exhibit the following characteristics: (i) hard, (ii)wear-resistant, (iii) brittle, (iv) refractory, (v) thermal andelectrical insulators, (v) nonmagnetic, (vi) oxidation resistant, (vii)prone to thermal shock, and (viii) chemically stable. Ceramics are thusknown to have excellent optical, mechanical, thermal, and chemicalproperties. In fact, polycrystalline ceramics exhibit extraordinaryproperties that cannot be reached by glasses due to its high strength.It is a combination of these properties that make ceramics veryversatile in their industrial use.

SUMMARY OF THE INVENTION

In an aspect of the invention, a method of forming transparent corundumceramics comprises milling corundum powder in aqueous slurry with beads.The method further comprises processing the slurry by a liquid shapingprocess to form a gelled body. The method further comprises sinteringthe gelled body in air and pressing the gelled body by hot isostaticpressing to form a ceramic body.

In an aspect of the invention, a method comprises: milling corundumpowder with BET of 15-24 m²/g in an aqueous slurry with corundum beads;processing the aqueous slurry by a liquid shaping process to form agelled body; sintering the gelled body in air at a temperature between1150° C.-1170° C.; and pressing the gelled body by hot isostaticpressing in Argon at temperatures between 1100° C.-1150° C. to form aceramic body.

In an aspect of the invention, a corundum ceramic body composed ofcorundum powder comprises the following properties: a hardnessHV10>2000; a 4 pt.-bending strength >600 MPa; an in-linetransparency >65% at 640 nm wavelength at thickness of 0.8 mm withpolished surfaces; a total forward transmission >80% at 640 nmwavelength at the thickness of 0.8 mm with polished surfaces; and athermoconductivity at room temperature of 27 W/mK.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is described in the detailed description whichfollows, in reference to the noted plurality of drawings by way ofnon-limiting examples of exemplary embodiments of the present invention.

FIG. 1 is representative of a processing flow and related equipment usedin accordance with aspects of the present invention.

FIG. 2 a shows a cross sectional view (SEM micrograph with magnification2000×) of the microstructure at the inner part of a transparent corundumceramics using corundum powder with small defects (pores).

FIG. 2 b shows a cross sectional view (magnification 1,000×) at the edgeof the same sample with large defects.

FIG. 2 c shows a SEM micrograph (magnification 10,000×) of a highstrength transparent corundum ceramics with inhomogeneous grain sizedistribution.

FIG. 3 shows a defect free microstructure (magnification 5000×) withhomogeneous grain size distribution according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The invention relates to corundum ceramics using corundum powder and,more particularly, to high strength transparent corundum ceramics usingcorundum powder and methods of manufacture. In more specificembodiments, the present invention is directed to transparent corundumceramics with homogeneous inline transmission defined by a difference ofinline transmission measurement at any point in an area of 20×20 mm²less than 1%. Advantageously, the present invention can be used in manydifferent applications ranging from aircraft and automobile materials touse in building industries and medical applications.

In an exemplary embodiment of the present invention, the corundumceramics of the present invention can be manufactured, for example,using the following processes:

(i) using corundum powder with BET of 17-21 m²/g;

(ii) milling the powder in an aqueous slurry with corundum beads;

(iii) processing the slurry by a liquid shaping process to form a gelledbody;

(iv) sintering the gelled body in air at a temperature between 1150°C.-1170° C.; and

(v) followed by hot isostatic pressing in Argon at temperatures between1100° C.-1150° C. to form the ceramic body. In embodiments, the hotisostatic pressing can also be in nitrogen or oxygen at about 1100°C.-1150° C.

In more specific embodiments, the method for production of transparentcorundum ceramics includes the use of corundum powder with a BET of17-21 m2/g; however, the present invention also contemplates otherranges. For example, the corundum powder can have a BET of 15-24 m²/g.In embodiments, the raw powder can be obtained from Taimei ChemicalsCo., LTD., Japan, under the trade name of TM-UF. The measurements, e.g.,BET of 15-24 m²/g, can be made with an ASAP2020 Physisorption Analyzer,Micromeritics, Norcross, Ga.

FIG. 1 is representative of a processing flow and related equipment usedin accordance with aspects of the present invention. As shown in FIG. 1,the corundum powder can be milled in aqueous slurry with corundum beads.For example, the milling can be performed by an attrition mill, e.g.,attrition milling processes, as shown representatively at referencenumeral 100. In alternative embodiments, the milling can be performed bya tumble or ball mill, vertical roller mill or the like, each of whichare represented at reference numeral 100.

In embodiments, the composition of the aqueous slurry comprisesdistilled water, with HNO₃ for stabilizing the pH of the corundumpowder. In embodiments, the pH should be about 4. In embodiments, HNO₃can be a stabilisator for the repulsion of the corundum particles. HNO₃is easily decomposable or other acids or steric stabilization withorganics like DOLAPIX or DISPEX. DOLAPIX is an alkali-free anionicpolyelectrolyte (Dolapix CE64) dispersant which is delivered as a 70 wt% aqueous solution and contained 30 wt % of the ammonium salt ofpolymethacrylic acid (PMAA-NH4). Dispex is a polyacrylate dispersantwhich is used to disperse inorganic materials. (DOLAPIX is a trademarkof DuPont.) A sintering aid, e.g., additive such as, for example, MgO,MgO precursors. Y₂O₃, La₂O₃, or TiO2, can be added to promotedensification during sintering.

In one example, the corundum beads can be about 2 mm in diameter;although other sized corundum beads are also contemplated by the presentinvention. For example, the diameter of the corundum beads can rangefrom about 0.5 mm to about 4 mm. In preferred embodiments, the corundumbeads are dense sintered corundum beads with sub-μm grain size. Infurther embodiments, 800 grams of milling beads can be used for 300 gramof powder; although it should be understood that other amounts and typesof milling beads can be used in the present invention, withoutlimitation to the above example. In embodiments, for example, the amountof milling beads can be dependent on the volume and type of mill. Forexample, the relation of powder to beads can be between 1:2 and 1:4.

In embodiments, the processing of the slurry can include a liquidshaping process. In embodiments, the processing of the slurry can beperformed by a liquid shaping process for about 2 hours in duration. Theprocessing can include different processing techniques as shownrepresentatively at reference numeral 105 of FIG. 1. For example, thecorundum powder, e.g., TM-UF raw powder, can be added to the slurry inan ultrasonic bath for 30 minutes, then the slurry can be added to 800grams of corundum milling beads with particle size 1-2 mm, for about twohours in an attrition mill at 1000 rpm.

The liquid shaping process can be one of many different ceramic formingtechniques, any of which is shown representatively at reference numeral105 of FIG. 1. In embodiments, the shaping process can be used forproducing advanced, high-temperature structural parts such as enginecomponents and the like from powders of ceramic raw materials. Forexample, the present invention contemplates a gelcasting process asshown at reference numeral 105; although other liquid shaping techniquesare contemplated by the present invention, e.g., “slip casting” or“coagulation casting” each of which are also represented at referencenumeral 105.

As should be understood by those of skill in the art, gelcasting is acolloidal processing method with a short forming time, high yields, andlow-cost machining, used to prepare high-quality and complex-shapeddense/porous ceramic parts. On the other hand, the present inventionalso contemplates slipcasting techniques. Slipcasting techniques areknown to provide a superior surface quality, and to achieve a higherdensity and uniformity in casting high-purity ceramic raw materials overother ceramic casting techniques, such as hydraulic casting.

In more specific embodiments, the liquid shaping process of the presentinvention can be performed with the following tools: ultrasonic bath,furnace with degassing process and a mold (glass mold) as shownrepresentatively at reference numeral 105. In embodiments, the degassingprocess starts with placing slurry molds (of any shape or dimension,depending on the characteristics of the final product) in a vacuumfurnace at about 20-25° C. (e.g., room temperature) with vacuuming theair inside the furnace until it reaches about 200 mbar. In embodiments,the vacuuming of the air inside can be provided until it reaches avacuum near the boiling point of the slurry. The boiling point for purewater is between 23.4 mbar at 20° C. and 31.7 mbar at 25° C. For theslurry the boiling point can be determined experimentally by reducingthe vacuum to the point when the first bubbles appear. Then, nitrogengas is inserted into the furnace until the furnace reaches atmosphericpressure, e.g., about 1000 mbar which is slightly below the standardatmospheric pressure of 1013.25 mbar, which depends on the altitudeabove the sea level and weather. Thereafter, the nitrogen gas isvacuumed until it reaches about 200 mbar. In embodiments, the vacuumingof the nitrogen inside can be provided until it reaches a vacuum nearthe boiling point of the slurry. The process of inserting nitrogen gasand vacuuming can be repeated two times, preferably. After waiting forabout 20-40 minutes at 200 mbar, the furnace is again filled withnitrogen gas. The temperature is then increased for the vacuum furnaceto about 40° C. to 80° C. and preferably about 60° C. for about 0.5 to8.0 hours, with a preference of about 4.0 hours. Afterwards, the furnaceis reduced to room temperature (e.g., 20° C. to 25° C.)

In embodiments, the shaping process results in a gelled body (formedfrom the mold) which is dried. The gelled body can be infiltrated withone or more precursors of metal ions solved in water to form coloredtransparent ceramics. After drying, organics are burnt out at about 800°C., and the body is sintered in air (using a sintering oven as shownrepresentatively at reference numeral 110 of FIG. 1) and hot isostaticpressed in argon (using a press as shown representatively at referencenumeral 115 of FIG. 1). The gelled body or the dried body or the porousbody can be infiltrated with solutions of salts like Cobalt(II) nitrate,Chromium(III) nitrate or Nickel(II) nitrate to form colored ceramics.

In embodiments, the ceramic body can be dried in air for around 1-14days, in order to remove any aqueous solution therefrom. For example, inembodiments, the gelled body can be sintered in air at a temperaturebetween 1150° C.-1170° C. to form a ceramic body, as represented byreference numeral 110 of FIG. 1. In more preferred embodiments,sintering can be performed, e.g., 5 K/min. (and even 1 K/min. to 10K/min.) to 950° C. and 2K/min. to final temperature. In furtherembodiments, the sintering can be, e.g., 1-10K/min. to finaltemperature, with a hold time of about 0 minutes to about 10 hours, witha cool down with 1-50 K/min. It should be understood that othertemperatures can also be used for sintering the ceramic body. Inembodiments, the sintering is performed in air.

In further embodiments, the hot isostatic pressing can be performed inArgon, Nitrogen or Oxygen at temperatures between 1100° C.-1150° C. Inembodiments, the hot isostatic pressing subjects the body to bothelevated temperature and isostatic gas pressure in a high pressurecontainment vessel as represented by reference numeral 115. Inembodiments, the chamber is heated, causing the pressure inside thevessel to increase. Pressure is applied to the material from alldirections (hence the term “isostatic”). When castings are treated withhot isostatic pressing, the simultaneous application of heat andpressure eliminates internal voids and microporosity through acombination of plastic deformation, creep, and diffusion bonding. Also,the hot isostatic pressing will increase the density of the ceramicmaterial, improving its mechanical properties and workability.

In embodiments, the pressing includes, e.g., heating with 2-5 K/min,with a pressure increase during heating, as representatively shown byreference numeral 120. In preferred embodiments, the pressing occurs forabout 8 to 15 hours at final temperature, e.g., 1100° C.-1150° C. underthe final pressure. For example, the preferred pressure is about 200MPa, with a range contemplated by the present invention of about 50 MPato 200 MPa.

In embodiments, the resultant ceramic body can have a thickness of about0.3 cm to 2.0 cm; although other dimensions are contemplated by thepresent invention. By way of example, the present invention contemplatesany shape with wall thickness between 0.5 cm to 3.0 cm, including hollowbodies and complex structures. In embodiments, the length and width ofthe ceramic body are only limited by the size of the mold and the sizeof the furnace. Also, by way of illustrative example, the finalcomposition of the ceramic body is greater than 99% Corundum. Inembodiments, the ceramic body can include dopants, e.g., Mg²⁺, Cr³⁺,Ni²⁺, and/or Co²⁺.

In embodiments, the processes of the present invention result intransparent corundum ceramics with homogeneous inline transmissiondefined by a difference of inline transmission measurement on polishedsurfaces or surfaces covered with liquid of same index of refraction atany point in an area of 20×20 mm² less than 1%. As should be understoodby those of skill in the art, homogenous inline transmission is providedby measurement, which can be performed with a light transmission meterLCRT2006, from Gigahertz Optik GmbH, Türkenheim, Germany, at wavelengthof about 640 nm and with defined aperture of 0.57° and a diameter of themeasurement area of 5 mm². It should be noted, though, that anyspectrometer with similar small aperture and a measurement spot ≦5 mm iswell suited for the present invention. It should also be noted that lessthan 1% is related to the difference of the results of measurements atdifferent points of the area, with the size of the area can be definedlarger, e.g., 100 mm²×100 mm² or 50 mm²×50 mm².

Once the processes are completed, the final properties of the ceramicbody can include, e.g.:

(i) Hardness HV10>2000, 4 pt.-bending strength >600 MPa;

(ii) In-line transparency at thickness of 0.8-1.0 mm with polishedsurfaces >65% at 460-640 nm wavelength and total forwardtransmission >80% at 460-640 nm wavelength;

(iii) For colored ceramics, the transparency can be lower by specificabsorption from metal ions like Co²⁺, Cr³⁺, Ni²⁺ incorporated into theceramics; and

(iv) Thermoconductivity at room temperature (e.g., 20-25° C.) of 24-28W/mK.

In additional contemplated embodiments, the in line transmission of thetransparent corundum can differ with different thickness and wavelength.By way of example, Table 2 shows in line transmission of the transparentcorundum with different thickness and wavelength. As shown in Table 1,the transmission increases with higher wavelength and lower thickness.

TABLE 1 Wavelength/ thickness (mm) 0.4 mm 0.6 mm 0.8 mm 1.0 mm 800 nm79.2 76.1 73.0 70.1 640 nm 75.3 70.5 66.0 61.8 460 nm 65.6 57.3 50.143.8

FIG. 2 a shows a cross sectional view (SEM micrograph with magnification2000×) of the microstructure at the inner part of a transparent corundumceramics using corundum powder with small defects (pores). FIG. 2 bshows a cross sectional view (magnification 1000×) at the edge of thesame sample with large defects. More specifically, FIGS. 2 a and 2 bshow microstructures made by field emission electron scanning microscopyof non-homogeneous (inhomogeneous) high strength transparent corundumceramic prepared with high purity corundum powder with BET ˜14 m²/g.Inhomogeneity can be represented by small defects or pores (see, e.g.,FIG. 2 a) and larger defects (see, e.g., FIG. 2 b). Normally thedistribution of such defects is irregular, i.e., see FIG. 2 a shows asegment from the middle of a sample and FIG. 2 b shows a segment of theedge of the same sample. At higher magnification the microstructurebetween the defects can look homogeneous; however such defects are limitnot only to transmission but also the homogeneity of the transmissionmeasured at different points of the sample.

FIG. 2 c, on the other hand, shows a corundum microstructure which isfree of defects but with inhomogeneous grain size distributionconsisting of finer grains and coarser grains. The inline transmissionof dense sintered corundum ceramics increase with decrease of grainsize. For high inline transmission grain size <1 μm is required andgrain size <500 nm is preferred. Inhomogeneous grain size distributiontherefore results in inhomogeneous inline transmission. The purpose ofthe present invention is to provide a transparent corundum ceramics withfine grain size realized by low sintering temperature and a homogeneousinline transmission realized by a defect free microstructure withhomogeneous grain size distribution. In embodiments, the sinteringtemperature should be high enough for full densification but as low aspossible to prevent further grain growth as described herein. Fine grainsize of the ceramics after sintering only can be realized with finegrain size of the ceramic powder.

It is known that sinter ability of nano corundum powders get worse withfiner particle size (respectively higher BET) of the powder because thenegative influences of the surface in relation to the volume of thepowder becomes much larger. Especially for liquid shaping processes, theelectro-static or steric stabilization of the surfaces of the particlescause a larger distance of the particles with increasing surface areawhich limits the processability. To overcome the larger distance of theparticles higher sintering temperatures are required. Finer particlesalso form stronger agglomerates which require higher sinteringtemperatures. Surprisingly, though, it was discovered that high puritycorundum powder with BET 15-25 m2/g can be processed with a liquidshaping process at lower sintering temperatures compared with a powderof BET ˜14 m2/g. Furthermore it was unexpectedly discovered that thedense sintered ceramics showed a much narrower grain size distributionresulting in a homogeneous inline transmission. Furthermore it wasdiscovered that the dense sintered ceramics manufactured in accordancewith the present invention, e.g., with the liquid shaping process usingpowder with BET 15-25 m2/g, was free of defects >150 nm over a range ofseveral centimeters resulting a homogeneous inline transmission.

It should be understood that high transmission demands low grain sizeswhich can be realized with low sintering temperature. (The sinteringtemperature and high pressure of the present invention are not known inthe literature, by the inventors.) The reasons that the presentinvention can achieve its advantages and final product are attributableto the specific powder characterized by specific BET and the specificprocessing with deagglomeration and liquid shaping. On the other hand,the literature notes a specific sintering process called SPS (sparkplasma sintering) or field assisted sintering (FAST) but with SPS thehomogeneity of the samples is poor because of thermal gradients in theequipment during sintering.

FIG. 3 shows a cross sectional view of the high strength transparentcorundum ceramics using corundum powder in accordance with aspects ofthe present invention. As shown in this cross sectional view, the highstrength transparent corundum ceramics of the present invention exhibitthe above noted homogeneous grain size distribution with fine grainsize.

Table 2, below, shows many of the properties/characteristics of thecorundum ceramic body in accordance with the present invention.

TABLE 2 CHARACTERISTICS OF CERAMIC BODY Purity >99% Thickness 0.3-2.0 cmTransparency yes Porous no Coloration possible High Hardness, HighBending yes Strength and Thermoconductivity Powder With Specific BET yesDeagglomeration yes Infiltration possible

The following are example processes in accordance with different aspectsof the present invention. It should be understood that any of theprocesses provided below are contemplated by the present invention andthat the above description is thus not considered limiting to thepresent invention.

Example 1

95.0 g of distilled water in a beaker were placed in an ultrasonic bathat pH 4. The pH was adjusted with HNO₃. Then 300 g corundum raw powderwith purity 99.995% and specific surface area BET of 17.5 g/m³ wasadded. An MgO precursor which forms 0.03 wt % MgO after thermaltreatment was also added to the slurry. The slurry was milled with 800grams of corundum milling beads with purity >99.95% and diameter ofabout 2 mm in an attrition mill for 2 hours at 1000 rpm. After milling,the beads were separated from the slurry and 9 g organic monomer and 3 gcrosslinker were solved in the slurry and a starting agent for the freeradical polymer chain reaction was added. Then the slurry was castedinto glass molds and polymerized under Nitrogen at 60° C. for 4 hours.The gelled bodies with a thickness of about 1 cm were demolded and diskswith diameter of 25 mm were cut from the rubber-like material and dried.The dried bodies were annealed in air with 0.5 K/min. to 800° C. for 2hours and sintered in air with 5K/min. to 950° C. and 2K/min. to 1170°C. for 2 hours. The sintered disk was hot isostatically pressed in argonwith 5 K/min. to 1150° C. for 15 hours. The transmission after grindingand polishing to a final thickness of about 0.8 mm was measured with aspectrophotometer Varian4000 at five different points of the sample witha shield with diameter 5 mm directly behind the sample. The transmissionat wavelength of 640 nm was: 63.07/62.59/62.68/62.59/62.75.

Example 2

95.0 g of distilled water in a beaker were placed in an ultrasonic bathat pH 4. The pH was adjusted with HNO₃. Then 300 g corundum raw powderwith purity 99.995% and specific surface area BET of 17.5 g/m³ wasadded. An MgO precursor which forms 0.03 wt % MgO after thermaltreatment was also added to the slurry. The slurry was milled with 800grams of corundum milling beads with purity >99.95% and diameter ofabout 2 mm in an attrition mill for 2 hours at 1000 rpm. After millingthe beads were separated from the slurry and 9 g organic monomer and 3 gcrosslinker were solved in the slurry and a starting agent for the freeradical polymer chain reaction was added. Then the slurry was castedinto glass molds and polymerized under Nitrogen at 60° C. for 4 hours.The gelled bodies with a thickness of about 1 cm were demolded and diskswith diameter of 25 mm were cut from the rubber-like material and dried.The dried bodies were annealed in air with 0.5 K/min. to 800° C. for 2hours and sintered in air with 5K/min. to 950° C. and 2K/min. to 1160°C. for 2 hours. The sintered disk was hot isostatically pressed in argonwith 5 K/min. to 1130° C. for 15 hours. The transmission after grindingand polishing to a final thickness of about 0.8 mm was measured with aspectrophotometer Varian4000 at five different points of the sample witha shield with diameter 5 mm directly behind the sample. The transmissionat wavelength of 640 nm was: 63.77/64.18/63.92/63.88/63.56.

Example 3

95.0 g of distilled water in a beaker were placed in an ultrasonic bathat pH 4. The pH was adjusted with HNO₃. Then 300 g corundum raw powderwith purity 99.995% and specific surface area BET of 17.5 g/m³ wasadded. The slurry was milled with 800 grams of corundum milling beadswith purity >99.95% and diameter of about 2 mm in an attrition mill for2 hours at 1000 rpm. After milling the beads were separated from theslurry and 9 g organic monomer and 3 g crosslinker were solved in theslurry and a starting agent for the free radical polymer chain reactionwas added. Then the slurry was casted into glass molds and polymerizedunder Nitrogen at 60° C. for 4 hours. The gelled bodies with a thicknessof about 1 cm were demolded and disks with diameter of 25 mm were cutfrom the rubber-like material and dried. The dried bodies were annealedin air with 0.5 K/min. to 800° C. for 2 hours and sintered in air with5K/min. to 950° C. and 2K/min. to 1170° C. for 2 hours. The sintereddisk was hot isostatically pressed in argon with 5 K/min. to 1150° C.for 15 hours. After polishing and grinding to a final thickness of 2 mmthe disk shows a homogeneous in line transmission according to theinvention.

Example 4

95.0 g of distilled water in a beaker were placed in an ultrasonic bathat pH 4. The pH was adjusted with HNO₃. Then 300 g corundum raw powderwith purity 99.995% and specific surface area BET of 17.5 g/m³ wasadded. An MgO precursor which forms 0.03 wt % MgO after thermaltreatment was also added to the slurry. The slurry was milled with 800grams of corundum milling beads with purity >99.95% and diameter ofabout 2 mm in an attrition mill for 2 hours at 1000 rpm. After millingthe beads were separated from the slurry and 9 g organic monomer and 3 gcrosslinker were solved in the slurry and a starting agent for the freeradical polymer chain reaction was added. Then the slurry was castedinto glass molds and polymerized under Nitrogen at 60° C. for 4 hours.The gelled bodies with a thickness of about 1 cm were demolded and diskswith diameter of 25 mm were cut from the rubber-like material and soakedwith 0.1 mol/l solution of chromium (III) nitrate in water for 4 hours.After drying the bodies were annealed in air with 0.5 K/min. to 800° C.for 2 hours and sintered in air with 5K/min. to 950° C. and 2K/min. to1170° C. for 2 hours. The sintered disk was hot isostatically pressed inargon with 5 K/min. to 1140° C. for 15 hours. After polishing andgrinding to a final thickness of 2 mm the disk shows a homogeneous inline transmission according to the invention.

Example 5

95.0 g of distilled water in a beaker were placed in an ultrasonic bathat pH 4. The pH was adjusted with HNO₃. Then 300 g corundum raw powderwith purity 99.995% and specific surface area BET of 17.5 g/m³ wasadded. An MgO precursor which forms 0.03 wt % MgO after thermaltreatment was also added to the slurry. The slurry was milled with 800grams of corundum milling beads with purity >99.95% and diameter ofabout 2 mm in an attrition mill for 2 hours at 1000 rpm. After millingthe beads were separated from the slurry and 9 g organic monomer and 3 gcrosslinker were solved in the slurry and a starting agent for the freeradical polymer chain reaction was added. Then the slurry was castedinto glass molds and polymerized under Nitrogen at 60° C. for 4 hours.The gelled bodies with a thickness of about 1 cm were demolded and diskswith diameter of 25 mm were cut from the rubber-like material and soakedwith 0.05 mol/l solution of cobalt (II) nitrate in water for 4 h. Afterdrying the bodies were annealed in air with 0.5 K/min. to 800° C. for 2hours and sintered in air with 5K/min. to 950° C. and 2K/min. to 1160°C. for 2 hours. The sintered disk was hot isostatically pressed in argonwith 5 K/min. to 1140° C. for 15 hours. After polishing and grinding toa final thickness of 2 mm the disk shows a homogeneous in linetransmission according to the invention.

Example 6

95.0 g of distilled water in a beaker were placed in an ultrasonic bathat pH 4. The pH was adjusted with HNO₃. Then 300 g corundum raw powderwith purity 99.995% and specific surface area BET of 17.5 g/m³ wasadded. An MgO precursor which forms 0.03 wt % MgO after thermaltreatment was also added to the slurry. The slurry was milled with 800grams of corundum milling beads with purity >99.95% and diameter ofabout 2 mm in an attrition mill for 2 hours at 1000 rpm. After millingthe beads were separated from the slurry and 9 g organic monomer and 3 gcrosslinker were solved in the slurry and a starting agent for the freeradical polymer chain reaction was added. Then the slurry was castedinto glass molds and polymerized under Nitrogen at 60° C. for 4 hours.The gelled bodies with a thickness of about 1 cm were demolded and diskswith diameter of 25 mm were cut from the rubber-like material and soakedwith 0.1 mol/l solution of nickel (II) nitrate in water for 4 h. Afterdrying the bodies were annealed in air with 0.5 K/min. to 800° C. for 2hours and sintered in air with 5K/min. to 950° C. and 2K/min. to 1160°C. for 2 hours. The sintered disk was hot isostatically pressed in argonwith 5 K/min. to 1140° C. for 15 hours; although the present inventionalso contemplated 1-10 K/min. After polishing and grinding to a finalthickness of 2 mm the disk shows a homogeneous in line transmissionaccording to the invention.

The foregoing examples have been provided for the purpose of explanationand should not be construed as limiting the present invention. While thepresent invention has been described with reference to an exemplaryembodiment, Changes may be made, within the purview of the appendedclaims, without departing from the scope and spirit of the presentinvention in its aspects. Also, although the present invention has beendescribed herein with reference to particular materials and embodiments,the present invention is not intended to be limited to the particularsdisclosed herein; rather, the present invention extends to allfunctionally equivalent structures, methods and uses, such as are withinthe scope of the appended claims.

What is claimed is:
 1. A corundum ceramic body composed of corundumpowder comprising the following properties: a hardness HV10>2000; a 4pt.-bending strength >600 MPa; an in-line transparency >65% at 460-640nm wavelength at thickness of 0.8-1.0 mm with polished surfaces; a totalforward transmission >80% at 460-640 nm wavelength at a thickness of0.8-1.0 mm with polished surfaces; and a thermoconductivity at roomtemperature of 24-28 W/mK.
 2. The corundum ceramic body composed ofcorundum powder of claim 1, wherein the transmission increases withhigher wavelength and lower thickness.
 3. The corundum ceramic bodycomposed of corundum powder of claim 1, wherein the corundum ceramicbody is transparent with homogeneous inline transmission defined by adifference of inline transmission measurement at any point in a definedarea less than 1%.
 4. The corundum ceramic body composed of corundumpowder of claim 1, wherein the corundum powder has a BET of 15-24 m²/g.5. The corundum ceramic body composed of corundum powder of claim 4,wherein the corundum powder has a BET of 17-21 m²/g.